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Why does an aircraft not spin out of control right after touch-down?
When a plane is landing, how does it not immediately start spinning out of control?
Given the massive amount of forward momentum and the fact that all the various braking forces (wheel brakes, spoilers, thrust reversers) are applied separately on each side of the plane, if there was even a small imbalance between the two sides wouldn't the plane want to start spinning (yawing) in that direction?
Is this something a pilot has to account for while landing or is there some physical effect that makes it Just Work?

The planes have steering too.

At least in the case of spoilers and thrust reversers, consider how the plane got as far as to the landing runway in the first place, and how that would be affected by any significant-enough-to-matter imbalances in the forces involved.

Because the plane has a front wheel. And no plane deploy any breaking until front wheel touches down, except some fighters deploy the parachute moments before touch down in the air.

I don't know the answer but I suspect it's "control systems engineering"...

@user3528438 It is perfectly possible to apply brakes with the nose wheel not on ground. Thrust reversers and spoilers, too! Positively controlled nose wheel touchdown is required of course.

"...is there some physical effect" Friction, I guess.

Your car has breaks either side but doesn’t spin out of control when you use them. Why would AB aircraft? The imbalance will be pretty small compared to the overall breaking and mass of the aircraft.

@Notts90 Personally I spin out of control every time I stop walking forwards.

how does it not immediately start spinning?

Physics, and engineering. But I guess that you would like a bit more detail, so let's try dive in.

wheel brakes

They tend to have a stabilizing effect by shifting some of the weight on the front wheel and allowing the pilots to control the aircraft with more effectiveness.

spoilers

These are automatically controlled by the Fly-by-Wire system, so if required they would be retracted.

thrust reversers

If they are functioning properly, the resulting force will go through the vertical plane containing the C.o.G. (but a bit below it), so they do not pose much of a problem.
Another thing you have not mentioned, but I am going to add, is crabbed landings (such as this one), since in these situations the aircraft DO tend to spin a little.
In these cases, in the timeframe between touchdown and braking, 2 things help the aircraft remain on the runway: the vertical tail (with the rudder), and the position of the main landing gear w.r.t. the center of gravity of the aircraft. As it can be seen in this image taken from the lecture notes of the course I followed (the title translates to "Braking run in crabbed landing"), normal tricycle gear aircraft tend to be stabilized by their own landing gear in this situation, while the same cannot be said for old school taildraggers.

So, what really could cause a problem? The answer is wind gusts (especially if the wind is perpendicular to the runway), uneven conditions of the runway (say, a puddle or a thin layer of ice on one side of the runway will cause one set of main gear to have less grip than its counterpart on the other side, causing a yawing towards the side with more grip), asymmetrical failures in the thrust reversers, excessive braking while also turning the aircraft to realign to the centerline (particularly a problem if the distance between the two main gears is not sufficient).
How does the aircraft remain on the runway despite these possible problems? At high speeds, thanks to the vertical tail; and at low speeds thanks to the front nose gear. At in-between speeds, the situation is slightly more complicated: the tail is not much useful anymore, while the front gear would risk breaking if used to correct too much. So, for example, a FbW system can help through the asymmetrical use of spoilers.
An example of what could happen, but seen during a take-off, is illustrated in this video (video kindly provided by MichaelK in chat).

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At landing the vertical stabilizer and rudder have a lot of aerodynamic force to counter any yawing motion. The pilot does have to account for any imbalances while landing.
There are many different sources of imbalances in yawing force at landing. As you mentioned, the brakes can be uneven. Engine thrust can be uneven. Crosswind is another large yawing force. The aircraft must be designed so that the vertical stabilizer and rudder are able to overcome these forces. Airliners are designed to be able to land safely with one engine out, with one-sided brake failure and crosswind up to a certain limit.
As the aircraft slows and the rudder loses effectiveness directional control passes to the wheels. Pilots use asymmetric braking and wheel steering to keep the plane on the center of the runway. It's at this point, later in the rollout, that problems are more likely to occur. This is when things such as skidding become more of a problem and runway excursions do sometimes occur. Anti-lock braking was designed to help prevent this.
Some aircraft, usually small tail-draggers, have no means of directional control other than the rudder. So that shows how important the aerodynamic forces from the rudder are to controlling the direction of aircraft both in the air and on the ground.

Small planes do spin out all the time. It's called a ground loop.
When a plane lands the pilot has to quickly do two things to keep the plane going straight: get ALL the wheels tight on the ground, and use the rudder and to steer the plane straight. If there is a strong crosswind, it can be quite tricky to do this in a small plane. The larger the plane, the more immune it is to crosswinds. One of the big dangers is overcompensating. Often a ground loop happens when the pilot oversteers.
BTW the other answers on the page have a lot of incorrect statements in them because none of the other posters are pilots. The main error they make is thinking that brakes are used to keep the plane straight. This is absolutely untrue. A pilot never applies the brakes until he has the plane going straight first, and then both brakes are applied simultaneously. A pilot never tries to steer with the brakes on landing--that would be a good way to get a ground loop started.
On nearly all planes from small to large, the brakes are at the top of the pedal (they are called "toe brakes"). What the pilot does is use just the balls of his feet when he lands to steer. For a small plane this can require rapid and accurate movements. Once he has the plane going straight and does not need to steer anymore, he shifts both feet to the top of the pedal and presses the brakes together to slow down. Once the pilot starts doing this, the plane generally keeps going in whatever direction it was pointed in, so it better be pointed straight. Sometimes the plane will not be pointed perfectly straight and the pilot ends up to the right or left of the centerline.

all the time (in your first sentence) is hyperbole (though it does happen occasionally).  That it can happen does not mean that "it happens all the time."

@KorvinStarmast It is ambiguous term, however, at many busy airports where a lot of beginner pilots fly, they probably have at least one ground loop every day or so. If you were to monitor all the airports in the United States, you would probably find there are at least 50-100 ground loops every day. So, I consider that "all the time". They are common events.

Tyler, as compared to the total number of landings, no, not common. You are overstating the case.  As stated, and without qualification or caveat, that statement is not true, and is a case of hyperbole.

The plane does not spin out of control because:

It takes the path of least resistance: on a dry runway, rolling friction is about two orders of magnitude less than sliding friction, so if the brakes are not applied there is a natural tendency to autocorrect.Your car also does not spin out of control in a corner, as long as the wheels don't slip.
If the plane lands with a sideslip velocity vector, once all three wheels make contact the nosewheel autocorrects: because of the way it is configured, the weight of the aeroplane pushes it straight. In small planes, the nose gear is inclined forward and is mounted in front of its axle, so that it lifts the aircraft up when turning. Same reason you can take your hands off of a bicycle handle bars.

In large planes, the long wheelbase provides stability. From the Wiki article for wheelbase:

Because of the effect the wheelbase has on the weight distribution of the vehicle, wheelbase dimensions are crucial to the balance and steering. For example, a car with a much greater weight load on the rear tends to understeer due to the lack of the load (force) on the front tires and therefore the grip (friction) from them.

And understeering is exactly what you want. From the Wiki for understeer&oversteer:

When an understeer vehicle is taken to the grip limit of the tires, where it is no longer possible to increase lateral acceleration, the vehicle will follow a path with a radius larger than intended. Although the vehicle cannot increase lateral acceleration, it is dynamically stable.
When an oversteer vehicle is taken to the grip limit of the tires, it becomes dynamically unstable with a tendency to spin out.

This self stabilising configuration is the main reason why an aircraft can be landed successfully with a crosswind and when not aligned perfectly with the runway. In these situations the aircraft needs to slam down, not touch down - transfer weight onto the wheels and the undercarriage configuration does the rest. The main wheels are placed apart wide enough to make sure that the aircraft cannot topple over, even with the most aft and highest centre of gravity.
Braking further pushes the nose wheel onto the runway, and increases the self-aligning force. All the braking system has to do is make sure that the wheels don't block: that removes the low rolling resistance. And then it needs to bring the aircraft to a standstill of course.
For correcting the lateral acceleration, the nose wheel steering is connected to the pedals after the main gear has detected ground contact, this enables the pilot to continue controlling the aircraft yaw when the velocity has become too low for the rudder to be effective. Pedal nose wheel steering has a much smaller range than nose wheel steering via the tiller.
The pilots have two brake pedals,one for each side, and can correct for a-symmetry via differential braking as well if required. The auto-stability of the touched down aeroplane is a must for successful landing, like aerodynamic stability is for successful flying. Airliners will never depend on active fly-by-wire systems for controlling landing stability - they may be helpful, but the aircraft must also be directionally stable when the systems are inactive, in any emergency for instance.

The control surfaces on the plane still work when the wheels are touching the ground. Also, planes have left and right toe brakes, unlike a single brake pedal in a car. The pilot can still steer with the control surfaces, tail rudder, using the pedals and also steer with the brakes by adjusting how much left and right side braking. When the plane is too slow for the control surfaces to be effective they use just the brakes to steer.
A helicopter can use the pedals to steer even with little or no forward/backward movement.


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Where can I find accident statistics on zero gravity flights?
I am investigating the option of booking a zero gravity flight with a commercial airline. But I am a nervous flyer.
Do any of the airlines offering this have a 0% accident / fatality record?

All of them, none have had accidents.

As with every flight the most dangerous part is driving to the airport... ;o)

If you are a nervous flyer then the vomit comet is not for you @Cloud.

What @GdD said:  If you are a self-identified nervous flyer, why in the world would you want a zero-G flight?  Perhaps a nervous flyer would be better suited to a flight school Intro Flight?

A reduced gravity flight is just as a normal flight but doing some stuff at a high altitude, typically between 24,000 and 32,000 feet. The stuff they do is this:

Image Source
Regarding the safety, FAA spokesman said:

Asked whether these parabolic flights will be as safe as normal
  commercial flights, [FAA spokesman], “If they operate within the
  parameters that we set out, yes, they will be safe.”

There have not been any accidents occurred for a commercial reduced gravity flight.

How is 34k feet zero gravity? Isn't that normal altitude?

@Cloud The "Zero-G" comes from the parabolic maneuver the aircraft is performing, not the altitude.

While the manoeuvres are different from normal airline operations, they are (probably) well briefed, (probably) flown by well-experienced pilots, and (if flown correctly) well within the approved capabilities of the aeroplane. That would make them, at the very least, not unsafe per se.

@Cloud:  So far from your comments, it seems that you are a nervous flyer, and don't even really know what a Zero-G flight is or how it works.  You even say you're wary of booking one.  Simple answer: Don't.  This experience is not for you until you understand it better and are comfortable with it.  There is a high probability you'll have a panic attack, or get violently sick, and spoil the experience for other passengers while having a horrible time yourself.  Please reconsider.

@Cloud just to clarify for you if you are not certain: the actual "zero G" part lasts only a very short time.  (30 seconds?)  The whole flight is only for the purpose of that short moment.   Surely you have been on a rollercoaster where you feel "light" as you go over the top?  That's all it is, they just do that, and make it last (say) 30 seconds.  Enjoy!

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Can RVSM safely be further reduced?
Since the world population shows no signs of slowing down its rapid increase, it's pretty safe to say that means we can only expect more concurrent flights as time moves forward.
Is it physically possible (with current aircraft models) to further reduce RVSM to say 500ft?
Since automatic altitude maintenance is required when flying in this airspace, I can't see why not.

There really isn't a need for it. Runway availability is a more limiting factor than cruise airspace. The areas where excess traffic is really a problem is not in RVSM cruise level airspace, it's in the vicinity of airports.

Reducing further to 500 ft separation is not safe with current aircraft models.
One reason is because of wake turbulence, this is described by ymb1's answer. Another, perhaps more compelling reason, not to use 500 ft separation is that it inevitably will lead to mid-air collisions.
The actual metal-to-metal spacing between aircraft in RVSM is often below 1000 feet. This is because of a number of factors:

physical aircraft height: e.g. an A380 is approximately 24 meter high. That is 75 feet.
deviations from the assigned altitude. For example overshoot/undershoots of 150 ft are acceptable when levelling off; brief deviations of 75 ft in cruise are not uncommon.
altimeter system error: the altimeter does not always indicate the exact pressure altitude; errors of up to 245 feet are possible for RVSM compliance.

It is easy to see that the sum of these components easily can eat more than half of your proposed 500 ft separation. If two aircraft on adjacent 500 ft separated flight levels have a bias toward each other, a collision will be the result. 500 ft separation is just not safe with the current aircraft.
In order to keep 1000 ft separation safe, aircraft operating in RVSM airspace are frequently monitored by RVSM regional monitoring agencies. They verify aircraft altimetry systems are operating within safe tolerances based off their transponder signals.
For the RVSM safety case (see ICAO DOC 9574), a total vertical error (TVE) of up to 300 ft can be tolerated by the monitoring agency.
The TVE is the sum of the Assigned Altitude Deviation (AAD) and the Altimetry System Error (ASE). The ASE may never exceed 245 feet.
The AAD is the result of small vertical disturbances and the altitude control system reacting upon that. Autopilots typically keep the AAD within 50ft, but some situations may degrade the height keeping performance.
Ideally the ASE is 0ft, but in reality there is often a bias in the altimeter.
ASE can be caused by:

Damage to static ports and pitot tubes
Pressure leaks in
pitot/static pipes
Air Data Computers out of tolerance
Poor paint finish in static port sensitive areas
Inadequacy of RVSM inspection procedures
Component life span
Non-optimised Static
Source Error Corrections
Pressure variation caused by skin
waviness effects

source: EUR RVSM RMA/Skybrary
If the RVSM Regional Monitoring Agency notices that an aircraft is trending towards the 245 ft limit (action is initiated at 180 ft error), they contact the aircraft operator so that corrective maintenance can take place. This way they ensure that RVSM with 1000 feet separation remains safe.
Below is picture of ASE measurements of a single aircraft that developed an altimetry error of 150ft within one year. The operator performed corrective action after being contacted by Eurocontrol's RVSM RMA. Thereafter the ASE was around the 0ft mark.

source: EUR RVSM RMA/Skybrary

"physical aircraft height: e.g. an A380 is approximately 24 meter high. That is 75 feet." and that is assuming the aircraft is level, roll or bank could presumablly increase the effective height significantly.

Great information. So, how do the RMA monitor the ASE? How do they know the system that they use to check altitude is 100% accurate?

@Cloud, that is a question on its own. In short, they measure geometric height using multilateration, they correct that using meteo data, they compare it to the altitude transmitted by the aircraft's transponder. The multilateration system monitors its own calibration by using looking at known reference transmitters that are at known locations. In addition there are many checks in the post processing of the raw data.

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Why would an aircraft do multiple runway laps before takeoff?
During a recent Ryanair, Boeing 737 flight, the aircraft went up and down the runway several times before takeoff, whilst modifying the flaps.
Why would this have been? Usually, I thought flaps and adjusted whilst stationary.
I was worried on the flight that they were 'testing' the engines somehow before takeoff.
Further info: I'm not sure what speed this was, but it was slightly faster than usual taxi speed.

Taxiing is typically done at idle or near-idle engine settings, so it's unlikely that they were somehow testing the engines by doing this. I don't have a ready explanation for what they were doing, though; taxiing along the length of the runway once isn't all that unusual in the grand scheme of things (it's called backtracking, and would typically be used on smaller airports where only one end of the runway is directly accessible by taxiway), and possibly back again if something is wrong after lining up for takeoff, but I fail to see what much would be gained by doing it more than once.

I guess the best thing I can think of is to ask: are you sure you actually were on the runway at the time? (And if so, on what basis did you reach that conclusion?) Some airports can be beasts to navigate along the taxiways. Is it possible that the pilots were in the process of taxiing to the runway, and for some reason the route they took was convoluted?

@MichaelKjörling This is a great point. No, I am not sure and it did seem there were a lot of turns as if we were navigating a maze...

It would be possible to go down the taxiway parallel to the runway, then be turned back by a problem related to the taxiway, and have to backtrack down the runway.  Rapid changes in the wind direction could cause the runway to change to operating in the oppposite direction.  Flight details (number/time/date) would help narrow down either of these issues.

When was this? You might be in luck and Flight Radar 24 may have your position on the ground which will show exactly where you taxied before takeoff.

@BDLPPL Takeoff was at 17:40 :)

Yes Budapest time. On Tues 1st May. I would rather not add to the question for anti-terrorism reasons. (The flight attendant already became suspicious when I asked him if the crew had performed this flight before, he said no...)

@Federico That's a BA flight. Ryanair only owns 737s so if the airline was Ryanair, it was certainly a 737.

@MJeffryes right, found a Ryanair one: https://www.flightradar24.com/data/flights/fr8207#113a78c4 it seems to have taxied even less

@Federico Yep! That was the flight.. ok it confirms I was just very paranoid. Sorry to waste your time.

First, congratulations on facing your fear of flying.
What you saw was nothing out of the ordinary (also, asking the crew things does not make you end up on terrorism watchlists).
From a passenger perspective, it's extremely difficult to judge your whereabouts. Taxiways and runways are difficult to distinguish, especially because your forward view is limited. The layout at Budapest, with runways far apart, does not help. Long taxiways may look like runways (taxiways may be very broad) unless you know exactly what you're looking for. I've invariably completely misjudged my position on the airfield as a passenger despite using the hours before boarding checking out the airfield layout.
Furthermore, the crew may elect to use a higher taxi speed on long taxiways, so when braking for a turn you suddenly realize how fast you were going. However, compared to the size of the plane this is still quite slow. Think of the feeling you experience braking for a bus compared to what you felt in the plane - a plane will taxi at what would be a fairly high speed for a bus. Due to the size of a plane and lack of external reference, cornering may feel exaggerated with respect to reality too - a 90 degree turn may feel like a 180.
Finally, the flaps are configured during taxi, no need to be stationary. This may be done in steps (a little bit before taxi, some more during) Controls can also be tested during taxi (full deflections to all sides). This is completely normal.
So, what you experienced as a sequence of things that are completely normal but feel very different inside the airplane than the way it looks on the outside.

Thanks very much.. although I have been worried by some of the answer here :/ :https://aviation.stackexchange.com/questions/51150/aviation-safety-statistics-the-math/51154?noredirect=1#comment133104_51154

even though I did the flight, it felt like hell at times so I am trying to do more research so I am more educated and therefore less anxious next time as I desperately do not want to give up on flying and travelling

@Cloud Statistics will never put you at ease if you can't put them into perspective.

@Sanchises runway markings are completely different from taxiway markings and they are visible through the side windows. Nevertheless one must know these markings, so I believe a regular passenger might get confused indeed.

@ElmoVanKielmo True. I've never confused a taxiway for a runway but I can imagine an average passenger would.

One other possibility not mentioned (and not relevant to Budapest, but this question might get views from people experiencing it at other airports) is backtracking.
On smaller airports it's somewhat common to not have taxiways connecting to the far ends of the runway, but rather a single taxiway connecting to the runway somewhere along its length (either the near the center or at one of the end points).
When aircraft need to take off on such a runway, they may have to turn onto the runway and drive part of the runway at taxi speed before turning around and starting their takeoff. Or after landing they may have to turn around and drive over the runway towards the taxiway.
This of course severely limits the capacity of the airport to handle traffic in volumes so it's mainly used at quiet places that don't get a lot of traffic, and sometimes at larger airports when for example a taxiway is closed for resurfacing.

That seems like a horrible design, but I'm sure there is a reason :)

@Cloud several reasons. Space is one (parallel taxiways effectively make the runway 3 times as wide). Closely associated is cost (less pavement to maintain). And many of these fields have existed since WW2 or before, in places where they were mainly used for the occasional transport or patrol aircraft and construction time was more important than future growth potential.

@Cloud another reason can be that the prevailing wind is always in the same direction. For the few days a year it comes from the other direction they just have to make do

Large airliners are not the only planes to taxi down and back.
I recently had to taxi down my home runway to scare some geese off the runway, and then keep an eye on them to ensure they stayed clear as we started our takeoff run.
Chasing deer away could be another situation.  A  low wing plane landing hit a deer over the weekend; killed the deer and dented the lower leading edge of the wing. 65-70 mph can do that.  Hitting engine/prop first I imagine would have made a lot more mess. No one in the plane was physically hurt. Mentally shaken up perhaps.

Please tell me you're not seriously suggesting an A320 was driving up and down the runway to scare off geese :/

No, something a little smaller, Cessna 177. Would have called to have someone come out if the operations area had been manned.

Taxiing up and down a runway was once thought to be a great way to lift the visibility limits when early morning fog would make an immediate take-off illegal. Until 1963, that is.
On September 4 of that year, a Swissair Caravelle used this trick with the engines at full power and brakes applied to slow the taxi speed. That worked to disperse the fog, but when the aircraft took off, the brake disks were glowing red hot. Inside the wheel well they cooled down only slowly and the heat crept into the surrounding structure. The magnesium wheel hubs heated up, too, and failed under the tire pressure when the hub material was heated to something close to its melting point. The resulting spray of shrapnel severed the hydraulic lines running through the wheel well and the still glowing brake disks ignited the hydraulic fluid, with catastrophic consequences.
That accident was one of the reasons for the use of fire-resistant hydraulic fluids in commercial aviation.

This doesn't answer the question at all

The answer does explain why you probably were not going up and down the runway though - beyond the usual "there are probably other planes wanting to use it"

@Cloud: The question is "Multiple runway laps before takeoff. Why?". The answer gives a reason. What is wrong with that?

@PeterKämpf Not really. this explains why it IS NOT practised anymore, not why it might still happen.

@Baldrickk if there are other aircraft waiting to use the runway you'd definitely NOT get clearance to take a leasurely drive over the runway, turn around at the far end, and do it again a few times just for fun.

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Is it possible to fly a B-24 under Bixby Bridge?
If that sounds like a ludicrous question it's because I believe it is, but here is why I'm asking.
My grandfather flew B-24s in WWII and there was something of a local legend that he once flew a crew-laden Liberator under Bixby Bridge (near Big Sur along the CA coast). Here is a short video of the bridge:

I say no way. Aside from the the idea of a trained military aviator so recklessly risking crew lives (not to mention court martial, etc.) straining credibility, I think such a feat is physically impossible for an aircraft of that size. But then, I'm not a pilot - I just pretend to be one on X-Plane. I have relatives (also not pilots) who are convinced the story is true. Those of you who fly IRL, what do you think?
Unfortunately my grandfather passed before I was born so short of breaking out the Ouija board there isn't really a convenient way to ask him!

If it fits –and I believe it does– of course it's doable...

@xxavier are you taking into account the terrain on the inland side that would have to maneuvered around?

Crazy is relative.  This was a different time.  Tex Johnson barrel rolled a 707 just because he could.  Not to say that it happened or that it didn't, but present day assesments of risk and sanity are likely to be clouded by a certain degree of contemporary squeamishness.  WWI pilots had a life expectancy of a few weeks on average.  By WWII this improved, but the kind of reckless disregard for risk, which today would get you properly disciplined, was then the kind of thing that was deliberately sought out and encouraged in pilots.  Who else would take a job where just surviving was a diceroll?

@J... Johnston's barrel roll was not very crazy at all. As he himself explained, he maintained 1g throughout the manoeuvre.

@MichaelK I'd like to hear Boeing's response to that if you were a modern day test pilot that pulled an unauthorized, impromptu barrel roll on a 777X prototype.  Even at the time he got a pretty stern response.  Anyway, the point is that lots of things that seem a bit crazy can actually be done reasonably safely, and certainly things we wouldn't dream of getting away with today had more leeway in the past. Most pilots today probably wouldn't take such risks simply because it's not responsible to do so, even if it's not terribly dangerous, while pilots of the past were somewhat more cavalier.

@J... You would get similarly disciplined if you did an unauthorised anything. The only difference doing it safely makes is to slightly mitigate the arse-beating you will receive for pulling stunts that you had not cleared with the people that are ultimately responsible for the aircraft. This is especially so since their bottoms would then in turn be ripe for being smacked around by the aviation authorities, wielding as their implement that big heavy volume that details Things You May And Things You May Not Do While In The Air.

@MichaelK Tex got a talking-to, but otherwise kept his job and was not disciplined. If it is your argument that a similar response would manifest today, I think I'll just let that speak for itself.

@J... The famous roll was executed on August 6, 1955. The FAA was established in 1958. And I do not think its predecessor the CAB (Civil Aeronautics Board) had any influence over such things at the time. In other words: the only people that had any say in the matter were Boeing themselves, and they retroactively approved the stunt. These days the people at Boeing will have to answer to the FAA if any shenanigans take place without prior authorisation. Point being: the safety of the stunt will be the difference between getting reprimanded or grounded, but you will get your arse smacked.

@MichaelK So... I rest my case.

"Aside from the the idea of a trained military aviator so recklessly risking crew lives (not to mention court-marshall, etc.) straining credibility..." You should read this: https://en.wikipedia.org/wiki/1994_Fairchild_Air_Force_Base_B-52_crash

@ceejayoz THIS. I was going to link that if no one else did. That pilot had a huge history of endangering his craft, crew, and surrounding civilians, and was basically never punished for it. Don't expect those risks to stop certain people, even in modern times, and don't expect military discipline to actually follow through in all cases.

With apologies for being extremely vague, memories and Google are failing me, but I remember visiting a museum in north Wales as a child, and it having a photo of someone flying a Spitfire under a really not very high bridge over an estuary. For which they definitely got disciplined, as it had a much smaller margin for error than a B-24 would have done on that bridge. Possible is different from happened, but I don't see any reason it couldn't have happened.

Re trained military aviators, this also: https://en.wikipedia.org/wiki/Cavalese_cable_car_disaster_(1998)

@ceejayoz: Around a decade before that (under a different commander) another Fairchild B-52 pilot noticed that his return route from a training mission over Alaska went fairly close to his home town--so he deviated just a bit from the flight plan, and buzzed his parents' house. That very day he got a chance to prove the he hadn't gained much weight in the last few years--his old uniforms with captain's bars still seemed to fit just about as well as the Major's uniform he wore when he took off. As far as I know, public knowledge of this was quite a bit more limited though.

@Jerry Coffin: So he may have gotten demoted for it afterwards, but he still did it, didn't he?  (Also, while I'm not that familiar with Air Force rank badges, in other service branches the rank insignia either pin on or are sewn on - no need to change uniforms.)

@jamesqf: Yes, he pretty clearly did it. As far as changing uniforms goes: on dress blues, the insignia are pinned on, so they're quick and easy to change. On nearly everything else, they're sewed on, and changing them is less trivial, and the outline of the old insignia would almost certainly be pretty visible. Immediately after a promotion, they're likely to overlook a minor violation of 35-10; after a demotion, people are likely to be much less forgiving. If I'd been in that situation, I'd have either found old uniforms our bought new ones, not taken chances on changing the insignia.

It's demolished now, but in 1941 an RAF Curtiss Tomahawk flew under a similar parabolic bridge near Winchester in the UK; the bridge was (incorrectly) then called Spitfire Bridge. The story is here - http://www.dailyecho.co.uk/archive/2003/10/31/Hampshire+Archive/5584779.Myth_of_pilot_s_bridge_stunt/  (edit - he nearly killed himself)

As others already pointed out, read about what bud Holland was allowed to do with a b52 and still allowed to fly

I'd say it sounds like a big little lie :-)

If experience in GTAV is admissible, I'd say this is doable but very difficult.

If the aircraft is very lightly loaded (manly to have room for the pilot`s cojones..) it looks entirely possibile.

your grandfather ROCKS dude.

Well said, @Fattie, I tend to agree, if I do say so myself! :) And I didn't even mention he was shot down over Europe TWICE, was beaten to an inch of his life by a mob of Hungarian villagers and spent the remainder of the war in a German detention camp!

It was certainly a different generation man.  That generation rocked.  My dear Mother, RIP, was quite old when I was born so, incredibly, she actually fought in WW2 as a very young teen.  Men were men, women were hot, and the enemy was badass. The only thing kids have today is phones and simulators.

Neglecting any potential power lines, I believe the manoeuvre is possible. I had to play around a little bit to get my simulation to work, though, showing that in real life, any attempt to underfly the bridge for real on the first try will be... risky. If I had to fly this for whatever reason, I would train the ground track at safe-ish altitude a couple of times before attempting the actual fly-under.
I am using a point-mass, trajectory-only simulation, which is accurate enough to study feasiblity of this type of manoeuvre, but not to determine exact values for required bank angles due to roll-in/roll-out turn radius variation. My aircraft isn't specifically a B-24 but a generic model; however the speeds, bank angles and descent rates I assume shouldn't be a problem for any aircraft of a similar size. Note that I am not modelling the climbout over the sea, as it doesn't really matter what rates are achieved, anyway, once past the coastline. I have used the bridge model mentioned by Tom and placed a B-24 model on my trajectory in Google Earth for a rough scale estimate.
This is how it looks:

My approach is towards the sea, as that will allow a safer recovery e.g. on engine failure than when diving towards the hills. [Later edit: Rates of climb in a B-24 trials report indicate on first looks that the necessary climb rates to clear the ridges when flying landward exceed the B-24's capabilities.] The manoeuvre is commenced at 150 mph indicated airspeed, in accordance with published best climb speed from the B-24 Flight Manual. I dive towards the ridge that has a path or road running along its top with 1000 ft/min descent rate, clearing it by approx. 120 ft. Then I roll into a 30-ish deg angle-of-bank right turn and push over into a steep 3000 ft/min dive. I clear the ridge in front of the bridge by approx. 100 ft (knowing that my right wing is lower, but still clear) and roll out pointing toward the bridge. My model gains approx. 14 mph during the 10-second steep segment of the dive as I can't generate enough drag, but if the B-24 behaves differently speed-wise it doesn't matter much, as the turn is nearly completed once speed has built up and the straight segment's geometry doesn't change measurably with speed. I clear the ridge underneath the bridge by some 50 ft and recover over the sea.
And... breathe.
PS: Anyone around who is tech savvy enough and would like to create the bridge for FSX?

This answer so far most thoroughly takes into account both the challenge of the terrain AND the performance characteristics of the aircraft. Thank you and well done...I may very well stand corrected!

well here's an attempt to follow that path... https://youtu.be/fY1I-20R734 ... though I think I would be tempted to come in from the ocean side with loads of beans and ride those ridges out... https://youtu.be/rN-vGZpGtF0

@TomH Well done! My reason to go towards the sea is that at any point you can safely break off the approach and climb away if anything happens. Coming in from the sea, after a certain point you’re committed under the bridge, even if all four engines quit (unlikely I know, but would you really want to bet your life on it with such a cool but, let’s face it, ultimately useless, manoeuvre)?

A Liberator has  wingspan of 34m, while the central span of the bridge is 110m wide, and 280ft high, so the aircraft will fit under the bridge.
Bixby creek approaches the bridge from the north-east, with a straight-ish approach of about 0.3miles. Approaching low from the north-east, and dropping down into the creek at the earliest opportunity might allow even a Liberator enough time to descend below the level of the bridge deck and recover over open sea, although the waves might be a little too close for comfort.
I wouldnt want to attempt it in a fully-laden B-24, but with a light fuel load and crew I'd say it's doable. Whether anyone ever did it is another story...

I hope those power lines weren't there if/when this was tried, though!

How high is the terrain 0.3m out? In other words, what descent gradient would be required to drop into the gorge on a straight approach and dip below the bridge? I‘ll try and have a go at modelling a trajectory one of these days - sounds like good fun! For info, a sensible manoeuvring speed for the B-24 seems to be around 150mph (from http://www.wwiiaircraftperformance.org/B-24/B-24_Flight_Manual.pdf).

Another aerial feat I would have expected to be impossible at Bixby: https://www.youtube.com/watch?v=QNfgu-bYEvc

I scanned that doc but couldn't find a performance chart or the climb tables. Maybe I should find my glasses first

@TomH I substituted best climb speed for manoeuvring speed as an approximation. The former is in the text for how to take off. Sadly, I didn’t find climb tables either.

Interesting... It looks possible under ideal conditions in a modern aircraft. (I attempted a basic simulated flight in google-earth flight simulator-vid below)
Edit: The service cables mentioned in the other answer would also represent a substantial problem, but they were missing from the model.
Assuming you can get through the gap, the main issue for the maneuver seems to be the ridge immediately behind the bridge in the valley, which is marked in red in this shot, (and then you've got a second ridge behind if you continue a straight climb out)

Obviously it puts a constraint on the climb out path if you approached from the sea, or the path of the descent if you started in-land.
It looks like you would have to follow a fairly specific path to avoid the ridge, though if you knew the terrain and the aircraft well, it would make it a lot easier.

If you started inland, a direct approach would include avoiding these ridges on the way down, and a fairly hairy check of the descent as you cross under the bridge... There would presumably be some ground effect benefit when attempting to obtain positive rate of climb over the beach/ocean.
Edit: adding the utility cables makes it look like a very small gap indeed!

I noticed that Google Earth has a flight simulator, so after a couple of crashes into mountains, I fumbled a path... (I hadn't worked out the flaps, or power keys at that point, so its totally with defaults as set when selecting flight sim mode)

Edit: following @Cpt Reynolds suggestion in his answer above, this is one route through;

Or alternatively, come in with as much speed as you can stomach, and just ride over the ridges;

It was done using the SR22 model (310 horses, cruise 185kts);

If you want to have a bit of fun trying the various approaches... I used this model of the bridge from sketchup, and imported into the google earth desktop app.

Wow...thank you for the effort that must have gone into this! Truly enlightening! It does raise a couple of questions in my mind: I'm guessing for this demo you used the F-16 model...? Do you think a B-24 could actually climb and make that steep turn fast enough to clear that first ridge? I'm also assuming a B-24 would have to pass somewhat lower under the bridge to clear the arch.

Nice answer!  it would be nice if you added some extra details to make it more relevant to the question though.

@JacobPerl It was flown using the SR22

@TomH I'm not sure it CAN be done properly in Google if there is no model resembling the perf. characteristics of the B-24 but nevertheless very interesting what you did here!

I guess the value with the sim is seeing if there is a path that could be flown under ideal conditions. Whether it could be done in a B-24 is something best left to experts

@TomH If I knew how I'd pop that bridge model into scenery in X-Plane which prob would have a model closer in specs :)

https://upload.wikimedia.org/wikipedia/en/0/09/Lancaster_Q_for_Queenie_flies_under_Sydney_Harbour_Bridge_1943.jpg lancaster bomber flies below sidney bridge. safety regulations were more reccomendations than rules :D

What the heck is that beige-colored ribbon in the first couple of pictures?

@jamesqf google earth renders the satellite imagery flat onto the terrain map, and so the whole thing gets flattened out, because the bridge isn't in the terrain map, which makes structures look very weird indeed. If you checkout something on google earth for which they have a model for, e.g. Sydney Harbour Bridge, you can see the structure with the ghosting of the flattened bridge splatted onto the ground below it. The images above with the bridge are a sketchup image imported into google earth

As for why a military pilot would do something like that, consider the context in which it was happening.
This would have been during WW2 training, when losses, especially to bomber crews, were very high.
Put a bunch of teenagers in flight training, where quite a few get killed, and when that's over, they're on a path to getting blown out of the sky over Germany, or forced down over Japanese territory where they may have their head cut off with a sword on the spot, and those teenagers might get a bit reckless in blowing off steam. We may die anyway, let's live life to the utmost until then. Think you can fly this big pig under that bridge? Let's find out...
The highest scoring US ace in WW2, Richard Bong, once flew a loop around the center span of the Golden Gate Bridge, and then buzzed downtown SF, getting below the tops of the buildings and waving at the people in the buildings that were at eye level. Granted, this was in a P38 and not a B24...

I'll admit my grandfather was known as a bit of a barnstormer in his time...did some crazy s*** at airshows and such!

this is actually a terrific answer, good one

As reirab mentioned there are currently utility lines that run half way up the arch. Looking at a modern picture you can see it's a communications line on the east side of the bridge with poles clearly visible on the north and south ends of the bridge.

Looking at pictures of when the bridge first opened in 1932 there were not any utility poles then. Pic1 Pic2
However, by 1937 there were old style telegraph/telephone poles visible in photos.

Original Picture and Alternate View
Unfortunately the old lines are too thin to see so it's impossible to tell from the photos exactly how much of the arch they block. I suspect the line was in roughly the same position as it is today based on the fact the utility pole spacing and how much the lines sag hasn't changed much over the last 100 years.
If you're lucky the building department that handles the Bixby creek area may have old building permits and diagrams for these lines which could potentially disprove the possibility of flying under this bridge. You could also post a question over on engineering.stackexchange.com to see if anyone can give a height range as to where lines would have been relative to the current ones.

Possible? Yes.
Smart? No.
Legal? Probably not now, may have been then (paraphrased as "need to maintain 500 foot clearance from things unless landing").

This.  Kinda like the B-25 that buzzed Florida Field during the Kentucky game forever ago on her retirement flight. See my answer to this question - https://aviation.stackexchange.com/questions/40945/has-anyone-ever-been-penalised-for-buzzing-a-control-tower/40950#40950

Unless things have changed, it's 500 ft from buildings or open-air assemblies of people, for civil aircraft.  (That's from memory, so I may be wrong.)  IDK if military rules are the same, but it's quite common to have military planes come a lot closer to you than that, out here in northern Nevada/eastern California.  I've been buzzed by jets while sitting on mountain peaks, and had a couple of C-130s fly over me at treetop level while out riding.  (Thought my horse was going to panic, but she just kinda shrugged, like "What the heck were those?")

Pilot's of the 174th ANG liked buzzing campus even though they supposedly did get 'punished' for it. They would fly from east to west over the quad which put them below height on some of the dorms on the east side of Campus. I remember two different occasions in 1991. One F16 flew low and slow and the other for about 10 seconds had the afterburner on but was higher up than 500'. It is my understanding that when in a civilian flight zone that they are supposed to follow civilian flight rules

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
Will the IFR Grade GPS be an requirement in the future?
This question is directly related to my other question so I have to ask it
I think its obvious why I ask this if you read that question but I will repeat myself again.
Station based navigation is literally dying and airspace is getting more crowded everyday.
So will the GPS be an FAA requirement in the future? I couldn't find it but I guess I am missing it.
I am just afraid of an future where people not finding approaches for airports which will limit them to the "backcountry" airports which still will have VOR DME and NDB.

So will the GPS be an FAA requirement in the future? It is a requirement in 2020 with the ADS-B mandate. *For certain airspaces...

@RonBeyer That's true, but I think the question here is about an approach-capable GPS, rather than an ADS-B position source. There are 'cheaper' ADS-B solutions out there that don't provide any GPS display in the cockpit.

Why do you think "backcountry" airports even have instrument approaches?

@Pondlife That's why it was a comment, I figured that he means the requirement for an RNAV GPS source to drive the approach indicator. Many ILS approaches have RNAV publications with them, and I assume that we'll see more and more ILS approaches move to RNAV as the primary approach rather than traditional radio's, although the "station based" navigation is not dying, it is just being replaced with other technologies like WAAS.

@jamesqf well. altough I dont know much about how much avionics the bush pilots have. I think they still use navigation aids for navigaiton and for landing at major airports like PABE.

@RonBeyer you are right. I was talking specifically about how pilots will aim towards the runways

"Station-based navigation is literally dying" this is a considerable exaggeration.

@0xdd exaggration? The FAA has been progressively closing them for a long time and they are finally planning to close almost all of them and WAAS is being improved! They will die in less than a decade!

That's cool, but there are large swaths of the world not regulated by the FAA (as well as admittedly small areas in the US) which will continue having station-based nav for a while to come.

"Almost all of them"?  No, about half of VORs in the Eastern US that were deemed redundant; coverage won't be materially affected.  And they're not taking out ILS systems either; they're just not putting in new ones (for GA).  They do seem to be killing NDBs outside Alaska, but that's no bug loss.

They are well on their way but it will never be a direct requirement per say. The regulations state

(2) Two-way radio communication and navigation equipment suitable for
  the route to be flown.

In many places a traditional VOR, DME, NDB or ADF may suffice as suitable navigation equipment however the FAA now has lower altitude GPS routes as well as many LPV approaches out there that are coming on line. Generally LPV approaches have lower minimums than their non precision radio beacon counterparts. Thus going into an airport that often sees weather may only be possible with an LPV capable (WAAS) GPS. In turn your operations may be severely limited without GPS moving forward.
The FAA will likely never institute a regulation that says something like all aircraft have to have IFR capable GPS' on board as thats not really how they regulate things. Eventually if they were to drop all ground based radio nav aids in order to be compliant with the above regulation everyone wishing to fly IFR would need to have an IFR legal GPS on board.

For the record "back country" airports are far more likely to have a GPS approach than a ground based radio approach as their may be no facility near by and GPS approaches can be simpler to maintain and add to an existing field.


text:
How can I legally organise a high school football game flyover in Delta airspace?
If a high school had given permission for a low pass over a football game, would the controllers also give permission? The school is about 3-4 miles away from the class D airport. Also, would you have to still stay at least 500ft above ground? I'm assuming you would still have to even if the school is allowing lower because that is the law.

You can get permission to fly lower, however its doubtful being a GA pilot that you would get clearance to do this over a populated stadium. By the way this falls under FAR 91.119(b) as an "open air assembly of people" and the minimum altitude would be 1000 feet above the highest obstacle within a 2000 ft radius, not 500.

That's true I forgot about that. I agree with you on having a little chance of getting permission to fly lower, but who would I have to talk to regarding this? Might just not do it at all.

Best thing to do would be to call the tower and ask, you may be outside the Class-D shelf anyway, and if that is the case you'll need to work with the local FSDO.

By the way when I say "call the tower" I don't mean when you're 2 miles out on the radio, I mean pick up the phone preferably a few weeks ahead, in case you need to work with the FSDO instead.

Oh of course, I knew what you meant! Thanks for the help

"I'm assuming [I would have to follow the law]". Yes. Yes you would.

Are you going to replicate "Good Luck Bokke!"

The simplest way to put this is that the school cannot give you permission to break the law.

First, the high school has no authority over the national airspace system so any 'permission' is meaningless, at least from the aviation point of view. I assume you want to coordinate with the school for timing and so on, but they can't approve or deny your flight, or tell you what altitude to fly at. (I'm ignoring any school disciplinary issues here, e.g. you're a student and the event is during class time.)
Second, the class D controllers probably won't mind provided that you give them some advance notice (call them a week or two in advance) and keep talking to them while you're in the air. Similar flights happen all the time in controlled airspace for photography and other reasons, so unless the stadium is right under final approach it shouldn't be a big deal.
Third, 14 CFR 91.119(b) says that the minimum altitude in this case would be (my emphasis):

Over any congested area of a city, town, or settlement, or over any
  open air assembly of persons, an altitude of 1,000 feet above the
  highest obstacle within a horizontal radius of 2,000 feet of the
  aircraft

(And don't forget that 119(a) requires you be able to make an emergency landing without endangering people on the ground.)
If you want to fly lower than that then you need a waiver from your local FSDO, but it's entirely at their discretion so you would need to convince them that you have a good, safe plan.
Finally, it's worth saying that there are a bunch of safety issues associated with doing things like this so if you do go ahead make sure you have a good plan and you stick to it. In particular, decide your minimum altitude and airspeed in advance and don't go below them under any circumstances.

Thanks for the help. I understand that the school cannot have any authority over me while flying, but like you said, it was more of a timing thing.

Wish I could give the "have a good, safe plan" line an extra upvote just on it's own.  I might also add in the term "detailed", they're going to want to know exactly what the pilot is doing and exactly what he plans to do if something goes wrong.

See the answers here. With that in mind, you can get waivers to get past these kinds of laws although that is up to the FAA NOT the high school's permission and frankly their permission means little if anything. The FAA's jurisdiction begins at 500ft in almost all areas and may be to the surface near an airport. While this is constantly up for debate in the courts, the main ruling stems from precedent set in 1946 during this case. Ron's comments are on point; if you can provide your location we may be able to offer assistance on who specifically to call. You can find a list of FSDO phone numbers here.

I'm in the central Texas area. Also thank you for being so helpful!


text:
How do groundspeed and IAS change during eights on pylons, and how should I manage them?
When performing eights on pylons I know that groundspeed (GS) reduces as we fly into the wind and increases as we fly with (or away from) the wind. This is the direct cause for the need to constantly change altitude (PA).
My question is in two parts:

As I descend into the headwind, why does my indicated airspeed (IAS) increase while my GS is reducing (and vice versa when flying with tailwind)? I have made a chart that shows XX GS = XX PA. When I look at my actual GS, I then look at my chart and find that I am at the correct PA for that GS but my IAS seems to spike in the descent and plummet in the climb.
Should I use the throttle to hold my IAS as it was in the entry?

It's been 50 years since I did eights on pylons, so take this with a grain of salt. As I remember, I did them at a constant power setting insofar as the throttle position was concerned. The rpm would vary because I was in an airplane with a fixed pitch prop. The change of IAS was due to the change in pitch of the airplane and thus to be expected, so I would say to leave the throttle alone after the initial power setting.  Any pitch change will cause a change in airspeed if the power is not changed, so an increase when pitching down is to be expected, and vice versa.

Eights on pylons are a completely 'eyes outside' maneuver, apart from occasional safety checks on altitude and airspeed. I'm not sure why you're trying to hit exact values, but assuming that you enter at a reasonable altitude and speed I wouldn't worry about them any more and rather focus on flying the maneuver correctly.

Answering the easy question first:
No, you should leave the throttle alone once you've begun the maneuver.
As for the other question:
Without seeing your technique, I suspect your IAS is changing because of your climb/descent rate
I don't know how quickly you're climbing and descending, but if you're trying to hit specific speeds at various points during the maneuver, it's likely that you're going to end up flying mechanically and abruptly. An eight-on-pylon should ideally be flown with your eyes mostly scanning outside, and only occasionally coming inside to reference your altitude and speed.
If you're trying to hit pre-computed numbers rather than looking outside and keeping the pylon on the lateral axis through smooth application of the controls, then my guess is that you'll be climbing and descending more than you need to, and you'll be doing it more abruptly than you need to, which may lead to large changes in IAS.
Have a look at the Airplane Flying Handbook, page 6-17:

With prompt correction, and a very fine control pressures,
  it is possible to hold the visual reference line directly on the
  pylon even in strong winds. .....  It is important to understand that variations in pylon position are according to the apparent movement of the
  visual reference line. Attempting to correct pivotal altitude
  by the use of the altimeter is ineffective.

(emphasis mine)
This part of the answer may be entirely incorrect, because I don't know what your flying technique is. If you can give us more information, we can give better answers.


text:
Climb/Descent or vectoring in other controller's airspace without point out
I'm an ATC trainee in an ACC.
Few days ago, I handed off an aircraft to the adjacent approach control, and the app. controller made it descend and vectored while it was still in our airspace; ACC's airspace. The STAR procedure starting from the handoff point says maintain present heading and flight level.
 It was not a dangerous situation and there was no trouble, but I'm just curious about this in terms of international regulations or recommendations.
I only found this on ICAO DOC 4444

4.3.3.1 When area control service and approach control service are not provided by the same air traffic control unit, responsibility for
  controlled flights shall rest with the unit providing area control
  service except that a unit providing approach control service shall be
  responsible for the control of:

a) arriving aircraft that have been released to it by the ACC;
b) departing aircraft until such aircraft are released to the ACC.

I want to know if there are specified regulations prohibiting 'climb/descent or vectoring in other controller's airspace without point out.'

Does your facility have a Letter of Agreement (or similar type of document) that releases control to the other facility within a certain distance o your common boundary?
This is a common practice, and such agreements should be covered in your training.


text:
Why do BWB designs typically put the engines in the rear?
A Blended Wing Body aircraft looks something like this:

In fact, every true BWB concept model I've seen puts the engines in the rear. Why is this? Can't they be hanging from the wings on pylons?
I'm envisioning a large cargo aircraft, if that maters. In fact I'm considering turboprop engines for even better fuel efficiency, so you could theoretically put one of them on the nose.
Is there something about BWB geometry that greatly favors rear engines? Wikipedia says the body can generate its own lift. Does that mean the airflow is faster over the top and therefore you get greater intake mass flow at the top of the tail area?

You should check Boundary Layer Ingestion, because of the huge size of the wing, the boundary layer will be very large, leading to large boundary layer momentum losses. By using this engines to accelerate this slower air, you minimize these losses

The airflow over the top of a BWB accelerates as it would do over the top of any airfoil, and by the time it reaches the wing's trailing edge, its speed has slowed back down, so any engines fitted at the back would see no advantage, from that point of view (although they might indeed have a beneficial effect on the wing's boundary layer).
There are at least two non-aerodynamic reasons for putting the engines at the rear of a BWB. The idea is for BWBs to be more environment-friendly than traditional jetliners, in terms of higher fuel efficiency and lower noise emissions.
In general, the fuel efficiency of a turbofan increases with the engine's diameter (compare the engines of the new A320 neo vs the old ceo), and you would expect a future BWB to have even larger engines than today's ones, which would be difficult to fit under an already thick wing.
Secondly, the engines at the back are also significantly quieter than under the wings, since the body of the aircraft shields people on the ground from the noise produced by the air intake (a major portion of the total noise emitted by a jet aircraft).


text:
Are frisbees and boomerangs technically gliders?
Frisbees and Boomerangs are heavier-than-air, unpowered, and fly through the air. So are they technically gliders?
They also rotate, so I wonder if these are actually some kind of rotorcraft-type gliders. I looked up all types of rotorcraft on wikipedia, which listed 4 things: helicopters, auto-gyros, gyrodynes, and cyclocopters. All of them were powered, so none of them were rotor-gliders. But then, auto-rotation in a helicopter might be considered gliding (with a pretty bad glide ratio).
One reason I ask is because I want to try to figure out the glide ratio or lift-to-drag ratio of these weird things. Not so easy to do empirically for a boomerang since it takes a weird ascending and descending path. How would you calculate that for a rotating assymetric airfoil?

Autogyro gliders are not unknown. For example, the Focke-Achgelis Fa 330  https://en.wikipedia.org/wiki/Focke-Achgelis_Fa_330

What is your definition of "glider"?

@Pondlife I've always heard it as heavier-than-air, unpowered flight, but never heard anything about rotation one way or the other.

@xxavier That's pretty weird. Surely a simple towed parasail is much more simple to deploy and pack up.

@Pondlife Btw, "What is your definition of glider?" is basically my exact question...just that the "you" refers to the "you the official organization/definition". The only caveat is that I'm specifically interested in the rotorary aspects of the term, cuz I've never heard them.

@DrZ214 Paragliders, which enabled fine control, weren't invented until 1952, and parasails, for ascending, first showed up in 1962.

@user71659 That is truly shocking. I learned something, that's for sure.

Rocks are heavier than air, unpowered, and fly through the air.  Some people define motorized parachutes as fixed wing aircraft, but that's a stretch in my opinion.  Definitions really only have meaning if there is a purpose for categorizing things, so are you just curious what others think?

It sure seems like it, based on FAA definitions.  "The Federal Aviation Administration (FAA) defines a glider as a heavier-than-air aircraft that is supported in flight by the dynamic reaction of the air against its lifting surfaces, and whose free flight does not depend principally on an engine. "
The FAA has also designated a paper airplane as a UAV ("drone").  And they do define a UAV as an aircraft  It might also be a rotorcraft depending on whether a whole vehicle can also be a rotor.

No, they are airfoils. Airfoils are the contoured shape of the wings that give a boomerang the proper amount of lift as it's launched and spinning through the air. They operate on the same principle as the wings and propellers of airplanes and helicopters. You might even say that a boomerang is a combination of a wing and propeller. Frisbees operate under two main physical concepts, aerodynamic lift and gyroscopic stability. When flying through the air, a Frisbee can be viewed as a wing, with Bernoulli’s Principle governing the magnitude of the lift force which keeps it aloft.

If you have a source with this exclusion in the definition, please share it. Otherwise I'm inclined to disagree. A flying wing is basically just a giant airfoil, but it's also a glider, isn't it? And yes there are some flying wings that don't have any active stability, e.g., Horton III: https://en.wikipedia.org/wiki/Horten_H.III

Not sure what you are asking. The early flying wings had a stability problem as their computers were not capable of correcting the in- flight osculations that occurred. This was overcome in the later aircraft like the F-114 with more advanced computers. Any powered aircraft can be a flying wing once the power is retarded and some glide better that others. Without power they are all flying wings, at least until they run out of altitude.

I was pointing out an example of an aircraft that is both a flying wing and a glider. (And I qualified it by saying it has no dynamic stability, so it is totally unpowered). The point was that flying wings can be considered "just an airfoil" too, but they are gliders (some of them). Therefore calling something just an airfoil is not enough to discredit it as a glider.

No. A glider is defined as an aircraft that is designed to fly long periods without an engine. The word that eliminates boomerangs and frisbees from this definition is ‘aircraft’ as an aircraft is defined as a machine capable of flight. Likewise, the phrase ‘for long periods’ also eliminates boomerangs and frisbees from the category ‘glider’ for obvious reasons. Both a frisbee and a boomerang can be more closely defined as a wing (or airfoil). When you look at the wing of an airplane or blade of a helicopter, those are what we call ‘airfoils’. An airfoil is a design that’s shape gives an object a positive lift to drag ratio. In the simplistic terms, the top of an airfoil is curved while the bottom is flat. Using Bernoulli’s equation, this shape results in pressure differences over the top and bottom portion of the airfoil as air flows over it. This is how a frisbee and a boomerang work. If you look at each of them from the side, you will notice a flat bottom and a curved upper side which, as previously mentioned, is what causes lift as they move through the air.

Any source for the "long periods" part? It's rather ambiguous anyway. A manned glider, if launched from the typical height of a paper airplane, will not glide for long periods. Conversely, a paper airplane launched from a high altitude will glide for long periods.


text:
How is windspeed scaled in wind-tunnel tests?
Let's say you have a scale model that is 1:10th the size of the real thing. You want to test this in a wind-tunnel.
Do you also scale down the windspeed 10x? Why or why not? I would think it has to be scaled down in order to get the right results for things like propeller tip speed.
Edit: I saw this question. It could be a duplicate, except I did not see an answer to my concern there. I want to know if windspeeds are scaled down in a windtunnel, whereas that one asks about how windtunnels are used in general and doesn't seem to mention any scaling of windspeeds.

In order to achieve the same flow conditions, you need to match several similarity numbers. The most important one expresses the ratio of inertial forces to viscous forces within a fluid and is called Reynolds number (Re):
$$\text{Re} = \frac{v\cdot l\cdot\rho}{\mu}$$
Nomenclature:
$\kern4mm v\kern6mm$flow speed
$\kern4mm l\kern7mm$characteristic length along flow path, like wing chord
$\kern4mm \rho\kern6mm$fluid density
$\kern4mm \mu\kern6mm$dynamic viscosity of the fluid
The next one on the list is the ratio between flow speed and the speed of sound in the fluid and is called Mach number (Ma):
$$\text{Ma} = \frac{v}{a}$$
Nomenclature:
$\kern4mm a\kern6mm$speed of sound
Others depend on the nature of the flow - if oscillating vortex shedding is involved, the Strouhal number needs also be observed, with elastic oscillations the Froude number or with heat transfer the Prandtl number.
In your case of a 1:10 model, the flow speed needs to be increased tenfold in order to match the Reynolds number. However, this will bring you in conflict with the Mach number, because the faster flow will probably already be supersonic. This problem is discussed at length in this and the linked answer, however, knowledge of the influence of flow speed is implicitly presumed.
Normally, in a wind tunnel test the flow speed is too low for correct scaling (which means that the viscous forces are higher in proportion to the inertial forces), and the consequences of this mismatch need to be covered by correction factors.
There is no need to scale forces correctly. Wintunnel models were hung on wires which were connected to scales to measure forces and now are mounted on a sting, the bending of which is captured by strain gages or an internal balance. Only the models used in free-fall tunnels for spin research will create as much lift as they weigh. The forces measured are used to compute coefficients, dimensionless numbers which are normalized for speed and size. When multiplied with the speed and size of the original, the (corrected) coefficients will yield the actual forces and moments.

Highly counter-intuitive. Woulda thought windspeed should reduce 10x too, like a 5 m chord meant for 100 m/s scaled down to 50 cm and 10 m/s, so the wind is moving 20 chord lengths / s in both cases. However, I see in the Lift equation, area would fall 100x, and v^2 would also fall 100x... Whereas mass would fall 1k times based on volumetric density, but Lift falls 10k times. Ouch. Your model would drop like a stone. I guess there are too many factors and they can never all be scaled correctly. Anyway, glad I asked this and glad it got re-opened and answered here.

@DrZ214: There is no need to scale forces correctly. Wintunnel models were hung on wires which were connected to scales to measure forces and now are mounted on a sting, the bending of which is captured by strain gages. Only the models used in free-fall tunnels for spin research will create as much lift as they weigh.

Yeah but if ailerons are just changing the camber of the wing, that's modifying lift, so you would get the wrong impulse or torque. Maybe I'm not understanding the actual purpose of windtunnels. I always imagined they tested the control surfaces and lift of the whole airframe at different angles of attack...at least, before the supersonic advanced stuff.

@DrZ214: Yes, of course they test the control surfaces as well. They measure the forces before and after deflection and compute a dimensionless coefficient from the results. This coefficient is normalized for wind speed and size, so it can be transferred to the larger and faster original.

It really depends on what data you need and the conditions you need it for.
Let's say you want to test a scale aircraft model in a wind tunnel in order to predict the lift forces on the full-sized aircraft during flight.
$Lift = q S C_L$
The dynamic pressure $q = \frac{1}{2}  \rho  v^2$
$S$ is the planform area
$C_L$ is the lift coefficient
$\rho$ is the air density
$v$ is the air velocity
What you really want to test for is the lift coefficient - you can use that to calculate the lift force during flight for a given dynamic pressure and planform area.
The lift coefficient will vary with Reynolds number and Mach number - these are the quantities you want to match.
So, you will not scale down the windspeed 10x - but it will be different from the flight-scale windspeed because the ambient conditions are different between the wind tunnel and flight at an altitude. The windspeed will be changed to give the correct Reynolds & Mach numbers.


text:
What aerodynamic characteristics arise from trailing edge sweep angle?
I drew 3 wing planforms, each with the same leading edge sweep. But they have different trailing edge sweep.

Let's just assume for the moment that all those wings have the same surface area, so that they all have the same total lift. What would be the different aerodynamic characteristics due to the trailing edge sweep angle?
Note: let's leave out considerations of control surfaces. Obviously the first wing would have the easiest implementation of control surfaces, but I want to focus on what the static wings do.
I looked into this and that related question. IMHO they're not quite the same thing as what I'm asking here.

Mechanical strength is a important factor. If you need more than 45 degree of sweep angle it's pretty hard to do with a simple parallel sweep (at least to make it strong and light), which contributes to why high speed aircraft prefers delta wings. But to reduce induced drag (for reasons to improve fuel efficiency) you want a high aspect ratio. For the same wing area, a high aspect ratio would translate a wide wingspan with parallel edges.

could you add some more details on what you keep constant and what not? How did you plan to keep the area constant, by increasing the chord length?

@rul30 The reason I mentioned constant area is so that each wing produces the same total lift. I want to compare sets of wings that have the same lift but different shape. I didn't have any specific way in mind to achieve that. I guess increasing chord length is the most straightforward.

Usually, the sweep angle is defined at the quarter chord line, and trailing edge sweep then follows from:

Wing area. Larger wing area means lower wing loading and associated structural weight, higher profile drag.
Aspect ratio. Higher aspect ratio results in longer span, higher structural weight, and lower induced drag.
Taper ratio. Higher taper ratio implies lower tip loading.
Quarter chord sweep angle. Higher sweep has beneficial effects on postponing transsonic shock waves, but detremental ones on stall behaviour and structural weight due to torque.

Seen in this perspective, trailing edge sweep is just a by-product of the four planform-defining entities. It may be desirable to have a straight trailing edge sweep for secondary purposes such as stowing the landing gear, and this may make the design team reconsider the combination of the four main parameters.
Image source
The three wing planforms drawn in the OP differ in all four defining parameters. Wing area and taper ratio decrease from up to down, aspect ratio and quarter chord sweep increase.
This article describes the aerodynamic design considerations for transsonic wings, and compares wings with varying sweep and thickness, same area and taper. The middle wing has a trailing edge sweep angle of close to zero, following more or less by coincidence from the definition of the main parameters.

maybe you could rephrase your answer. The constrains by DrZ214 might be arguable, but in his question the area and the projected span did not change (the chord is not scaled properly).

@rul30 They are drawn that way though, that's why I've included the NASA article drawing.

I can't understand everything in the graphs. What is on the y-axis of the 2nd graph? What does t/c mean? Why is the left graph labeled (a) Lambda = 47 degrees even tho it has 3 lines (which I'm assuming correspond to the 3 wings you drew, each of which have different Lambda)? I found this graph in your link but could not find these answers in the surrounding paragraphs.

@DrZ214 Y-axis of 2nd graph is $C_{Dmin}$ as well. t/c is wing thickness as a fraction of local wing chord: the article is on critical Mach and the two ways of delaying that are reduction of wing profile thickness, or increase of wing sweep. Graph (a) is three different thickness ratios at constant sweep, graph (b) is three different sweep angles at constant thickness.  All is to illustrate that the considerations are with wing sweep measured at quarter chord: middle drawing quarter chord sweep  $\Lambda$ = 35°, the trailing edge sweep is close to zero as a coincidence, not as a design goal.


text:
What is the typical weight distribution ratio between nose gear and main gear?
On an airplane with tricycle gears, the main gear supports most of the weight and is therefore the strongest. The nose gear prevents the nose tipping down, since the center of gravity is forward of the main gear.
What is the typical ratio of weight distribution between the nose gear and all main gears?
For the purpose of the question, I'm mostly interested in airliners, although an answer comparing that with GA would be useful as well. I understand that the weight distribution varies with loading, so perhaps a range would do (e.g. on most airliners the max nose gear loading cannot exceed X:Y).

Related: Page 235 of this A350-900 Airport planning document, or this screenshot. Logically this ratio is the half the ratio of gear distances to the CG, and in the case of the A350-900 about 5.5 (depending on the balance). I believe the largest force on the main gear is during the touchdown.

@mins Yes, while some very light 'greaser' landings might be exceptions, a normal landing is definitely the largest force on the main gear, as the force has to accelerate the entire mass of the airplane upwards with a magnitude equal to its prior downward motion over a rather short distance (the compression of the tires and gear struts.) The main gear has to be designed to deal with forces much larger than what it encounters while just sitting around on the ramp.

kevin, could you perhaps specify roughly what type of plane you're wondering about (e.g. large transport category, light GA, etc.?) I'd guess this varies quite a bit between different classes of airplanes.

Also, this will depend on how you've loaded the aircraft. If you load it with the CG far enough aft, the nose gear supports no weight at all. However, this is usually a bad idea.

Related: http://aviation.stackexchange.com/questions/16447/why-is-the-main-landing-gear-of-the-cessna-caravan-placed-so-far-forward/16455#16455

Look at the sketch of the recommended gear placement relative to the center of gravity (cg) from this answer:

We can calculate the load distribution for both extremes. For simplification we use the same angle of 60° for the nose wheel in both cases:

Main landing gear 6° aft of cg: $\frac{\tan 6°}{\tan 60°}$ = 6%
Main landing gear 20° aft of cg: $\frac{\tan 20°}{\tan 60°}$ = 21%

Add some dynamic loading from the application of wheel brakes. In this answer it is shown that the horizontal braking force is less than 30% of the maximum static weight of the aircraft, and applying this knowledge to the nose gear at 60° from the cg adds 0.3$\cdot\cos 60°$ = 15% of the maximum static load of a fully loaded aircraft to the nose gear load.
In extreme cases the nose gear would carry almost half of the aircraft mass, but then rotation would be impossible. In case of the Go-229 it was 45%.

Semi-finished Go-229 V3 (picture source)
On the other extreme you find nose gears with zero static load which were only intended for taking the braking forces.

Zeppelin Staaken R VI (picture source)

Jenkinson recomends at least a static load of 8% W on the nose-wheel to give reasonable steering forces. Also not more than 15% W static load on the nose-wheel, as more than this will make it difficult to rotate the aircraf. Furthermore it is importent to provide an adequate reverse stabilising moment for backward towing and general stability.

authors: آذر فلاح پور


text:
Can an airliner complete a flight with one pack inoperative?
I was travelling as a passenger on a 747-8. Around 5 minutes after we left the gate, the captain said we had to return to the gate due to "technical issues with the air-conditioning". The mechanic troubleshot the problem for around 45 minutes, during which the air-conditioning was completely off (but the fans were still running), and it was getting hotter by every minute. Finally, the announcement came and told us the problem could not be repaired on spot. We had to switch plane to continue the journey.
Most airliners have two "packs": left and right. They provide air pressurization and temperature control to the cabin.

Is it feasible to complete an entire flight with one pack inoperative?
If #1 is true, is it legal to takeoff with one pack known to be inoperative? (i.e. does the Minimum Equipment List allow this?)
If one pack fails shortly after takeoff, would the pilot turn around and land, or continue to the destination?
What alert message did the pilots likely got in their cockpit in my case?

The 747-8 has 3 packs, they are very powerful.

The fact that an airliner can be dispatched with some system inoperative (according to MEL) does not necessarily mean that the airline will dispatch it. The airline must consider not just this flight, but the overall disruption to the schedule, so especially if spare plane is available, it is often better to switch and fix it here then risk having to fix it somewhere else where getting a spare plane might be a bigger problem.

The 747-8 has three packs, and it is feasible to complete the flight with one of them inoperative (in case of aircraft with two packs, cabin altitude can be maintained win any one). The following is taken from the Minimum Equipment list of 747-8:

Screengrab from 747-8 MEL
As can be seen, flight is possible with some limitations. In case of a failure in-flight, the flight crew would follow the applicable checklist and decision will be taken whether to continue the flight or not (as altitude is limited). In case of failure, the appropriate warning message will be displayed (the pack, sensor, valve etc.)


text:
How do helicopter pilots practice auto rotation?
How do helicopter pilots practice auto rotation, and specifically, the landing phase?
Timing the flare as ground is approaching seems critical to a successful auto rotation landing. Yet I can't figure out how this can be practiced safely.

In my airfield, it's frequent to see police helicopters, some of them big ones, in practice flights. They often do autorotations, reporting that over the radio, but I've never seen them landing that way. They give power and resume normal flying after gliding 2-3 km in autorotation...

If you put "auto rotation" into a YouTube search, you can see how it's done: the instructor without warning just yanks the throttle backwards and down you go. Either you stick the landing, or the instructor takes over before you crash. The same way you practice any emergency in any kind of airplane: you do it "for realz", but with a guy who knows what he's doing beside you.

Training wise: Big helicopters, use sims to do full-down autos. During initial training, they usualy recover to a hover a few feet above the ground. Once you're working on your CFI, then you start taking them to the ground. This is what I've observed watching helicopters training.

Are you asking specifically about the flare?  If you are asking how in general, then it's a duplicate

@JörgWMittag You do not close the throttle without warning.

@Simon yes I'm specifically interested in the flare.

Yet I can't figure out how this can be practiced safely. Good thing you aren't in charge of flight training then. Using the Jet Ranger we did autorotation practice back in '82, (run on) and they have been doing it ever since. Did it before then, also.  (TH-57 Jet Ranger/Bell 206).  Here's the current training manual.

@KorvinStarmast Was your first auto in a Jet Ranger?  You had it easy then ;)

@Simon Yes indeed, the Navy chose well with it's ab initio trainer, we also flew hueys but were prohibited from the full auto at that stage. I understand that some other light helicopters are quite sporting in that mode ...

@KorvinStarmast I remember my first "follow me through" demonstration in a 206 and the instructor saying, purposefully slowly, "oh, it looks like the engine has quit, yes, the RPM is dropping, we'd better enter autorotation, there we go......now for the flare - looks about right, yes, ease her back, there's the RPM coming up, lets level, yep, that looks level, ok ease up the collective to cushion" and so on.  Coming from an R22, I was more used to as fast as you could go, "lever down, stick back, pedal in.....flare NOW, a bit more, level out, contact".  I was amazed by how much time I had.

@kevin Autos in the R22 were nerve wracking.  In the 206, they were just fun!

You just fly your profile. Initially its with an instructor, but eventually you move on to training by yourself/copilot.
You can practice full autos (to a landing), but some airframes practice to a hover or in early stages. In the TH-57B (Bell 206), you can take it a full power-off landing during training since it is light enough. However, moving up to the TH-57C you do power-on recoveries, where you run the twistgrip back up to full open while you're in the flare. You then initiate the recovery to arrive at a standard 5-foot hover. The C model has significantly more weight, which is the main reason.
In the MH-60S its all power-on, recovery to a hover. Except we do not go to idle like in the 57s, we just rapidly lower the collective to get the needle split. Same process in practicing your scan, initiating the flare, and then pulling in power at the appropriate height once you've leveled. Finish at 10 feet with some forward speed. (Rumor is that Army does full practice autos in their 60s, but never confirmed that). Navy prohibits it in the MH-60S and almost certainly in the MH-60R as well. The explanation I received is that the airframe is meant to absorb the impact similar to your car during a full auto, and thus full practices can structurally compromise the bird.
Source: MH-60S driver

Tom, we didn't do full autos in the SH-2F either, and most of the SH-60B community came from there.  (No full autos in the 60B either when I was an FRS instructor, early 90's).  The CH-60S/MH-60S were informed by the previous experience. The general reason we didn't do that, and didn't do night autos to a field either, was that the Wing (and higher) had done a risk assessment back in the late 80's (before I got out into 60's) and due to the simulator being "good enough" chose not to hazard the inevitable prang that practice autos to touch down eventually have happen when someone has a bad day.

I may be able to find an Army Black Hawk guy who can confirm the "not to a touch down" point, I did a bunch of full autos in a Black Hawk sim ... may take a day or two to find the right old comrade on that one.

It seems that you already understand what an autorotation is but for the benefit of other readers, I'll recap what the flare  is all about.
There are two really critical parts of an autorotation.  The entry and the flare.  If you get the entry wrong, then there is no autorotation.  You become a passenger all the way to  the scene of the crash.
If you get the flare very wrong (not enough speed when you flare or flaring far too high), you become a passenger all the  way to the scene of the crash.
If you make a mess of the flare, perhaps plenty of speed but too early, or too late, or not enough speed but flaring at a good height, then you will survive.  You might not get to use the helicopter again and you might spend some time in hospital but you're not going to  hit the  ground with enough energy to ruin all your mushy internal parts.
The reality is that most GA and low hour pilots faced with a real autorotation have a high chance of messing up the flare, and the subsequent touch down, damaging themselves and the machine but surviving.  The accident statistics and reports confirm this.  Chopping off the tail and rolling over are common outcomes of real autos.
In training and practice, the idea is that you walk away and get to use the helicopter again.
So, what height is the "right" height for the flare?
The POH emergency procedures section will also detail the height but, it will use slightly vague words such as "about 50 feet".  This is because most practice autorotations end with you at 50 feet with around 60kts airspeed and the nose straight - "the ideal autorotation".  The are many factors which affect this, mainly weight and wind (ideal ones always finish into wind).
Take a look at the "height velocity" diagram for a typical GA/sport helicopter.

(Source Rotorwandwing.com)
The shaded areas show the combinations of height and velocity from which a successful safe landing cannot be accomplished in autorotation.  When you flare, you are going to be close to the ground.  You have no choice in that!  When you start the flare, you want to be well away from the shaded areas of the diagram.  A normal autorotation aims for an entry into the flare with 55-65 kts airspeed.  Look at the diagram again and note that being at around 40-50 feet at that speed is well away from the shaded areas - bang in the middle of the "safe corridor".  This is the area you are aiming for.  At this point, you have enough energy, both kinetic and potential, to land safely.
You can see why flaring too high, too low or with not enough speed can lead to a bad weekend if you make a mess of it or, a permanent removal of flying privileges if you get it really wrong.
Hopefully this gives you a good idea of what the "right height" means.
When doing ab-initio training, you are gently led into autorotations. Your instructor will demonstrate one to you very early.  If you are clearly confident and not nervous, maybe on the first lesson which my instructor did.  Indeed, at my school, every single dual flight included at least one demonstrated or practice autorotation so you get a feel for the height early on.
You will also, perhaps every time (I think this is true in the US) finish an autorotation with a power recovery.
I chose that video because there's a good illustration in it of how to judge the height - and given that "about" is as good as the POH gives you, it's all about judgement.  In this video, you can see everything.  Watch it and look straight ahead.  Do not look at the ground, let your peripheral vision do the work. Notice how as you get closer to the ground, it seems to be coming towards you at a constant rate then suddenly, it seems to rush towards you - the "ground rush"?  That's when you start the flare.  You already know from the power recoveries you've practiced and the demonstrations what the correct height looks like so you know roughly when to flare but that sudden ground rush is the sure sign that now is the time to start slowing down.
What you are aiming for is to arrive at 0 rate of descent at or just above the ground so that a little bit of collective can be used to cushion the touch down.  With some wind on the nose and good airspeed, it is possible to bleed off most of the descent rate and ground speed with the flare.  A skilled pilot might not even use collective until the last couple of feet for a really gentle 0-0 touchdown.
There are a lot of different kinds of autorotations and some advanced ones don't use flare at all - or very little flare from low speed.  Really skilled pilots in light helicopters can go more or less vertically down with just enough airspeed to keep translational lift.
In summary, good judgement which is arrived at from the POH reinforced with demonstrations and practice.
Note to other rotary pilots:  I know that using the HV diagram is not how it's trained but it's useful in my opinion to understand what the correct height is.